# std::decay

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< cpp‎ | types

Defined in header `<type_traits>` |  |
template< class T > struct decay; |  | (since C++11)

Performs the type conversions equivalent to the ones performed when passing function arguments by value. Formally:

- If `T` is "array of `U`" or reference to it, the member typedef `type` is `U*`.

- Otherwise, if `T` is a function type `F` or reference to one, the member typedef `type` is std::add_pointer<F>::type.

- Otherwise, the member typedef `type` is std::remove_cv<std::remove_reference<T>::type>::type.

If the program adds specializations for `std::decay`, the behavior is undefined.

### Member types

Name | Definition
`type` | the result of applying the decay type conversions to `T`

### Helper types

template< class T > using decay_t = typename decay<T>::type; |  | (since C++14)

### Possible implementation

    template<class T>
    struct decay
    {
    private:
        typedef typename std::remove_reference<T>::type U;
    public:
        typedef typename std::conditional<
            std::is_array<U>::value,
            typename std::add_pointer<typename std::remove_extent<U>::type>::type,
            typename std::conditional<
                std::is_function<U>::value,
                typename std::add_pointer<U>::type,
                typename std::remove_cv<U>::type
            >::type
        >::type type;
    };

### Example

Run this code

    #include <type_traits>

    template<typename T, typename U>
    constexpr bool is_decay_equ = std::is_same_v<std::decay_t<T>, U>;

    int main()
    {
        static_assert
        (
            is_decay_equ<int, int> &&
            ! is_decay_equ<int, float> &&
            is_decay_equ<int&, int> &&
            is_decay_equ<int&&, int> &&
            is_decay_equ<const int&, int> &&
            is_decay_equ<int[2], int*> &&
            ! is_decay_equ<int[4][2], int*> &&
            ! is_decay_equ<int[4][2], int**> &&
            is_decay_equ<int[4][2], int(*)[2]> &&
            is_decay_equ<int(int), int(*)(int)>
        );
    }

### See also

remove_cvref (C++20) | combines std::remove_cv and std::remove_reference (class template)
`implicit conversion` | array-to-pointer, function-to-pointer, lvalue-to-rvalue conversions

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